1. Field of the Invention
The invention relates to a receiver circuit for receiving and forwarding data signals.
2. Description of the Prior Art
Receiver circuits having integrated input amplifiers have been generally known for a relatively long time and are used for a wide diversity of circuit purposes and applications. Solely as regards the general background, reference shall be made to European patents EP 869 615 B1 and EP 869 614 B1 which each describe single-stage input amplifiers. Although applicable to any desired receiver circuit having input amplifiers, the present invention and the problem on which it is based are explained below with reference to input amplifiers for semiconductor memories.
There is increasingly a requirement, in modern computer and software applications, for ever larger quantities of data to be processed in an ever shorter period of time. Large-scale integrated memories, for example DRAM memories, are used to store the data. In order, then, to meet the requirement for an ever higher speed when processing data, it is necessary, in the case of such a semiconductor memory, for the data to be written to the memory and read out from said memory again in an appropriately rapid manner.
As development advances in the field of integrated circuits, the operating frequency thereof also rises, with the result that the data can be processed in an appropriately rapid manner. In addition, semiconductor memories which are specially designed for high data rates also exist. One representative of such a semiconductor memory is the so-called DDR-DRAM memory, where DDR stands for “double data rate”. Whereas in conventional semiconductor memories write and read operations are performed only upon the rising edge or the falling edge of a clock signal, in DDR semiconductor memories data are read out from the semiconductor memory and are written to the semiconductor memory again both upon the rising edge and upon the falling edge of the clock signal. A double data rate is thus realized.
The data are read out from the semiconductor memory and are written to the semiconductor memory via an external interface which typically contains one or more input amplifiers. Since, on account of the high frequency, in particular, a very large quantity of data is read out or written via this interface, there is the particular requirement for these quantities of data to be shifted as effectively as possible, that is to say as optimally as possible in the available time windows. In this case, it is essential to comply with the so-called set-up and hold times which are defined, inter alia, by the maximum possible read-out speed and thus the efficiency when transferring data from and to the semiconductor memory.
DDR semiconductor memories and other applications use signals in which, together with the data signal, a reference potential is also simultaneously transmitted and injected into the input amplifier. FIG. 1 shows the timing for reading data into a known input amplifier, as is used, for example, to read in data from a DDR semiconductor memory.
V_EXT is used here to denote the data signal which is transferred to the DDR semiconductor memory, the voltage level of this external data signal V_EXT respectively being based on the reference potential V_REF which is concomitantly transmitted at the same time. The two signals V_EXT, V_REF represent the item of input data. The external data signal V_EXT is received, amplified and forwarded by the input amplifier circuit. In this case, the external switching point ES is defined as the point of intersection between the externally injected data signal V_EXT and the reference potential V_REF, the external switching point ES, by definition, being the same for a rising edge and a falling edge of the external data signal V_EXT. Forwarding and amplifying the external data signal V_EXT results in the internal data signal V_INT, the internal switching point IS1, IS2 resulting from the point of intersection between the internal data signal V_INT and an internal switching threshold V_IX which is typically defined by the circuit which is connected downstream of the input amplifier.
The problem with this is that, in the case of conventional input amplifier circuits such as the input amplifier described at the outset, the delay time for falling edges (tpf) and rising edges (tpr) of the data signal V_OUT is, inter alia, a function of the reference voltage V_REF and the operating point of the input amplifier. However, the reference voltage V_REF is typically subject to external fluctuations, for example fluctuations in the supply voltage and the temperature. In addition to external parameters such as the supply voltage and the temperature, the gain Av also depends, inter alia, on internal parameters, for example technology-dictated process fluctuations when fabricating the integrated input amplifier circuit. Without additional measures, this gives rise to different delay times tpf, tpr which result in different respective internal switching thresholds IS1, IS2 for a falling edge and a rising edge of the internal data signal V_INT. This considerable dependence impairs the set-up and hold time for writing data.
This read-out operation is effected by latching the data, the data which are coded in the data signal being determined by changing the logic level from 0 to 1 and vice versa, that is to say by a falling or rising edge. By virtue of the fact that there are different signal propagation times (delay times) tpf, tpr for a falling edge and a rising edge, a longer set-up and hold time thus also results overall, which directly results in the speed for writing the data decreasing if all of the transmitted data are to be read in reliably. In practice, this leads to significant impairment of the overall data processing efficiency since a worst case scenario for the set-up and hold time must be taken into account for reliably reading in and writing data, which overall, however, is at the expense of efficiency.